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Carbohydrate Polymers 118 (2015) 119–125

Contents lists available at ScienceDirect

Carbohydrate Polymers

j ourna l ho me page: www.elsev ier .com/ locate /carbpol

tructure and properties of moisture-resistant konjac glucomannanlms coated with shellac/stearic acid coating

ueqin Weia,b, Jie Pangb,∗∗, Changfeng Zhanga,∗, Chengcheng Yub, Han Chenb,ingqing Xieb

Shandong Key Laboratory of Storage and Transportation Technology of Agricultural Products, Jinan 250103, ChinaCollege of Food Science, Fujian Agriculture and Forestry University, Fuzhou 350002, China

r t i c l e i n f o

rticle history:eceived 20 May 2014eceived in revised form 26 October 2014ccepted 10 November 2014vailable online 15 November 2014

a b s t r a c t

A series of moisture-resistant konjac glucomannan films were prepared by coating shellac/stearic acidemulsion on deacetylated konjac glucomannan films (dKGM). The effect of stearic acid content on struc-ture and properties of the coated films were investigated by field emission scanning electron microscopy(FE SEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet spectroscopy (UV), water vapor per-meability (WVP), water uptake, water contact angle, and tensile testing. The results revealed that shellac

eywords:eacetylated konjac glucomannanhellcahe coated filmater vapor permeabilityoisture-resistant properties

in the coating adhered intimately to the surface of dKGM film, and provided a substrate for the dispersionof stearic acid which played an important role in enhancement of the moisture barrier properties andmechanical properties of the coated films. The WVP of the coated films decreased from 2.63 × 10−11 to0.37 × 10−11g/(m s Pa) and the water contact angle increased from 68◦ to 101.2◦ when stearic acid contentincreased from 0 wt% to 40 wt%, showing the potential applications in food preservation.

© 2014 Published by Elsevier Ltd.

. Introduction

It is well known that the natural polymers are of high safetynd environmentally friendly. KGM, a high-molecular weight het-ropolysaccharide, is composed of �-(1→4) linked d-glucose and-mannose in the molar ratio of 1:1.6 with a low degree of acetylroups at the side chain C-6 position (Li, Ji, Xia, & Li, 2012; Maeda,himahara, & Sugiyama, 1980). With good film-forming propertiesnd health-protective functions, KGM shows promise in packagingnd coating of food (Jia, Fang, & Yao, 2009). However, the applica-ions of KGM films are limited by its hydrophilic and sensitive to

oisture, due to a lot of hydroxyl groups in the repeating units ofGM (Xu, Luo, Lin, Zhuo, & Liang, 2009). In our previous work, basedn the study of KGM structure (Jian, Zeng, Xiong, & Pang, 2011; Jian,ang, Yao, & Pang, 2010), the KGM/curdlan blend films, with the

ater vapor permeability (WVP) value of 19.53 × 10−11 g/(m s Pa),ave been prepared (Wu et al., 2012), showing better moisture-esistant properties than pure curdlan [21.50 × 10−11 g/(m s Pa)]

∗ Corresponding author at: Shandong Institute of Commerce and Technology,516 Travel Road, Caishi, Licheng District, Jinan 250103, China.el.: +86 531 86330135; fax: +86 531 86330125.∗∗ Corresponding author. Tel.: +86 591 83705076; fax: +86 591 83705076.

E-mail addresses: pang3721941@163.com (J. Pang), zcf202@163.com (C. Zhang).

ttp://dx.doi.org/10.1016/j.carbpol.2014.11.009144-8617/© 2014 Published by Elsevier Ltd.

but far from LDPE [0.19 × 10−11 g/(m s Pa)] (Phan The, Debeaufort,Luu, & Voilley, 2008). These results motivate us to carry out furtherresearch about the moisture-resistant films based on KGM, usingan alternative method.

One way to achieve a moisture-resistant film with barrieragainst WVP is to coat or add some extra hydrophobic componentsto the film substrate, and then produce the coated film (Cao, Deng, &Zhang, 2006; Lu & Zhang, 2002; Lu, Zhang, & Xiao, 2004) or bilayerfilm (Anker, Berntsen, Hermansson, & Stading, 2002; Debeaufort,Quezada-Gallo, Delporte, & Voilley, 2000). In term of the coatedfilms, the regenerated cellulose films coated with interpenetrat-ing polymer networks (IPN) coating exhibit good water resistivity,unfortunately, the natural polymers in IPN coating are less than30 wt% (Cao et al., 2006), failing to thoroughly utilize renewableresource. In term of the bilayer films, casting lipid onto the pro-tein or polysaccharide films (Anker et al., 2002), the greasy surface,waxy taste and potential rancidity problem of lipids curb their usein food applications (Phan The et al., 2008), despite their WVP havesignificantly decreased. Hence in this work, one of the major tasksis trying to select other natural hydrophobic polymers as coatingmaterials and coat them on KGM film.

Shellac, obtained by refining the secreton of Kerria lacca, isone of the few natural polymer allowed for coating purposesin food, owing to excellent film-forming, high gloss, and poorpermeability to gases, acid and water vapor (Byun, Ward, &

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20 X. Wei et al. / Carbohydrat

hiteside, 2012; Farag & Leopold, 2009; Soradech, Nunthanid,immatvapirat, & Luangtana-Anan, 2012). It has also beensed for the moisture protection of drugs in the pharmaceu-ical industry and found to decrease the WVP of edible filmsLimmatvapirat et al., 2004; Luangtana-Anan, Nunthanid, &immatvapirat, 2010; Soradech, Limatvapirat, & Luangtana-nan, 2013; Wang, Chen, & He, 2008). Shellac is comprised ofolyesters and an ester with polyhydroxypolybasic acids, and

nsoluble in water but soluble in alcohol and alkaline solutionLimmatvapirat, Limmatvapirat, Puttipipatkhachorn, Nuntanid,

Luangtana-Anan, 2007; Luangtana-Anan, Limmatvapirat,unthanid, Wanawongthai, Chalongsuk, & Puttipipatkhachorn,007). It is well known that stearic acid, as a component of waxsed by nature to create superhydrophobic lotus leaves, has beensed in the field of moisture barrier due to its low surface energyWu, Pan, & Li, 2010). Moreover, with low cost, low toxicity,nd biocompatibility, stearic acid could embed into regeneratedellulose film to form a micro–nano binary structure, resultingn the improvements of hydrophobicity (He, Xu, & Zhang, 2013).owever, shellac/stearic acid composite coating has seldom been

eported (Phan The et al., 2008; Soradech et al., 2013; Wang et al.,008).

In this paper, in view of seldom reports on the prepara-ion of the coated films based on KGM, we attempt to prepare

series of moisture-resistant deacetylated konjac glucomannanlms (dKGM) coated with shellac/stearic acid, in which stearic acidontent ranges from 5 up to 60 wt%. The effect of stearic acid contentn the coating layer on interfacial and surface structure, mois-ure barrier properties, and mechanical properties of the resultingoated films were investigated in detail.

. Materials and methods

.1. Materials

Purified konjac glucomannan (Mw = 3.4 × 105 g/mol) was pur-hased from San Ai Konjac Food Co., Ltd. (Sichuan, China). Shellacfood grade) was provided by Kunming Xilaike Bio-technology Co.,td. (Kunming, China), which was used without further purifica-ion. The stearic acid, Na2CO3, NaBr and 98% ethanol purchasedrom Sinopharm Group Chemical Reagent Co., Ltd. (China) were ofnalytical grade. Polyethylene glycol 400 (PEG 400) was chemicalure with purity ≥99.7%. Polyethylene film (PE, Top Group Co., Ltd,hina) and qualitative filter papers (Hangzhou Tezhongzhiye Co.,td, China) were bought and used directly.

.2. Preparation of the coated films

Deacetylated konjac glucomannan films (dKGM) were obtainedfter solubilization of 1 g of KGM flour in 100 g of deionized water atoom temperature for 2.5 h under a 300 rpm stirring. Subsequently,.3 g of Na2CO3 was added, keeping stirring for 15 min. The film-orming solution was kept for 30 min under the same conditionsithout stirring prior to be spread onto polystyrene Petri dishes (PS,

50 mm × 25 mm) to provide a sheet with the thickness of about mm and evaporated in an oven at 55 ◦C for 5 h to obtain dry filmsith the thickness of about 30 �m. Then, the films were washed

xhaustively with distilled water to remove the residual Na2CO3nd evaporated again in an oven at 45 ◦C for 2 h.

The shellac/stearic acid coating was prepared by dissolving2 g of shellac particles together with desired content of stearic

cid, such as 0 wt%, 5 wt%, 20 wt%, 40 wt% and 60 wt%, in 100 gthanol under a 300 rpm stirring for 60 min at room temperature.hen, 1.2 g of PEG 400 were added it under a stirring speed atbout 5000 rpm (PT 3100 D, KINEMATICA, Switzerland) for 15 min.

mers 118 (2015) 119–125

The resulting mixture solution, namely coating, which preparedfrom different stearic acid content (0–60 wt%) were denoted SSA0,SSA5, SSA20, SSA40, and SSA60, respectively. The coating wasthen coated on the both sides of dKGM by brush, and cured at75 ◦C for 15 min. The coated films were referred to as dKGM-SSA0,dKGM-SSA5, dKGMSSA20, dKGM-SSA40, dKGM-SSA60, respec-tively. Before various characterizations, the resulting films werekept in a conditioning desiccator over saturated NaBr solutionwhich fixed 57% RH at room temperature for more than one weekto ensure the equilibrium of the water in the films.

2.3. Scanning electron microscopy

Film samples were observed by field emission scanning elec-tron microscopy (FE SEM, Nova NanoSEM230, FEI, America). Thefilms were frozen in liquid nitrogen, immediately snapped, andthen dried for the FE SEM observation. The surface of the films wassputtered with platinum for 2.5 min and then observed with 5 kVaccelerating voltage.

2.4. Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FT-IR) was carried outwith a FT-IR spectrometer (VERTEX 70, Bruker, Germany) in thewavelength range from 4000 to 400 cm−1. The powdered and driedsamples were obtained, and the test specimens were prepared bythe KBr disk method.

2.5. Ultraviolet spectroscopy

Ultraviolet spectroscopy of films, with thickness of about 38 �m,were carried out with a ultraviolet–visible spectrophotometer(Lambda 800 UV/VIS, Perkin Elmer, America) in the wavelengthrange from 200 to 800 nm.

2.6. Water vapor permeability determination

Water vapor permeability (WVP) at a relative humidity differ-ential was measured using China National Standard GB 1037-88(1989) modified by inversing its RH differential, homologous to theASTM E96-80 (1980). All films used were fixed between two Teflonrings on the top of the glass cell containing distilled water of whichthe relative humidity is 99% at 25 ◦C. Test cell was introduced into aclimate-controlled chamber (GSP-9160MBE, Shanghai BoxunshiyeCo., Ltd., China) regulated at 57% RH and 23 ◦C; 11% RH and 23 ◦Cor 57% RH and 38 ◦C. Thus, three relative humidity and tempera-ture differentials (gradients) were used: 57–99% and 23 ◦C; 11–99%and 23 ◦C; 57–99% and 38 ◦C, which mimic a product exposed todifferent ambient.

Test cell was periodically weighted up to a constant weightvariation rate. WVP [g/(m s Pa)] was calculated according to thefollowing equation:

WVP = �m × d

A × t × �p

where �m (g) is the amount of water vapor movement across thefilm (weight loss); d (mm) is the film thickness; A (mm2) is the areaof the exposed film; �p (Pa) is the actual difference in partial watervapor pressure between the two sides of the film specimen; t (s) isthe time during which a stable weight gain occurred. At least threereplicates of each film type were tested for WVP.

2.7. Water uptake measurement

The water uptake of the films was measured according to theprocedure described by He, Xu, and Zhang (2013). The films used

e Polymers 118 (2015) 119–125 121

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ere preconditioned at 38 ◦C for more than two weeks and weighedW0). After immersing in distilled water for an expected time, thelms were blotted with filter paper to remove the excess waterarefully on the surface and weighed (Wt). The water uptake ratioas calculated according to the following equation:

U(%) = Mt − M0

M0× 100

.8. Water contact angle testing

Water contact angle was measured and calculated in dynamicode on a Data Physics Instrument (OCA20). One drop of water

2 �L) was put on the surface of the films with an automatic pistonyringe and photographed at time 30 s.

.9. Practical application: storage of wheat flour in the coatedlms

Wheat flour was used to test the practical moisture resistance ofhe coated film dKGM-SSA40 and PE film. As package material, theKGM-SSA40 and PE films were used to seal the dried vessels (W0).hen, the vessel which contained wheat flour was weighed (W1)nd introduced into climate-controlled chamber regulated at 34 ◦Cnd 99% RH for several period and weighed (W2) according to ISO12-1998. The water content ratio of wheat flour was calculatedccording to the following equation:

C(%) = W1 − W2

W1 − W0× 100

.10. Mechanical properties

The mechanical properties of the films were measured by aexture analyzer (EZ-TEST, Shimadzu, Japan) according to ISO 527--1995 (E) at a speed of 5 mm/min. The specimens (10 mm width)sed were 70 mm long. Before characterizations, the resulting filmsere also kept in a conditioning desiccator over saturated NaBr

olution which fixed 57% RH at room temperature for more thanne week. And all measurements were made under the same con-itions (about 67% RH at 25 ◦C controlled by air-condition) owingo strength data are related to the environmental temperature andumidity.

. Results and discussion

.1. Structure

Fig. 1 shows FT-IR spectra of the SSA0, SSA5, SSA20 and SSA40oating. The molecule structure of shellac was reported as aolyesters consisting of a number of acidic and hydroxyl sidehains (Wang et al., 2008). The broad bands at around 3434 cm−1

ere attributed to the OH vibrations from acidic and hydroxylide chains. The two peaks at around 2930 cm−1 and 2861 cm−1

ere ascribed to the C H stretching vibrations of CH3 and CH2roups, respectively, while the bending C H vibrations of CH3 andCH2 groups could be found at around 1372 cm−1 and 1415 cm−1,

espectively. The strong bands at 1729 cm−1 and 1640 cm−1 wereharacteristics of C O groups from acid and ester structures,espectively. For the coating SSA5 and SSA20, the addition of steariccid to shellac caused an decrease in coating absorption intensity,ndicating that shellac and stearic acid had miscibility (Zhang, Xue,

o, & Jin, 2006, chap. 3). While for SSA40, the location, band widthnd shape of the absorption band tended to be similar to that ofhellac, implying that there are almost immiscibility between shel-ac and stearic acid in ethanol.

Fig. 1. FT-IR spectra of coating for SSA0, SSA5, SSA20, SSA40.

Fig. 2 shows the FE SEM images of the coated films surfaceand their coating layer cross-section. The dKGM-SSA0 surfacewas smooth (Fig. 2a). The surface of the dKGM-SSA5 appearedmicroscale wrinkle, leading to a slightly rough surface (Fig. 2b).With 20 wt% stearic acid addition, the roughness of the coated filmsurface became more obvious, exhibiting pitting (Fig. 2c). However,for dKGM-SSA40 and dKGM-SSA60, their surface appeared someextra black spots (Fig. 2d and e). Apparently, it was aggregatedstearic acid that made black spots observed.

The micrograph of monolayer SSA0 coating displayed in Fig. 2f.After mixed with 5 wt% stearic acid, the cross section of the coat-ing layer appeared a lot of uniform microgaps (Figs. 2g). The gapstended to be larger when the content of stearic acid reached 20 wt%(Figs. 2h). It suggested that shellac and 5 wt% or 20 wt% stearic acidin the coating layer mixed uniformly. This can be explained bythe fact that shellac and stearic acid were soluble in ethanol butwith different solubilities. The solubility of shellac and stearic aciddecreased in the process of solvent evaporation (Xie et al., 2004),leading to phase separation, that is, microgaps. On the other hand,shellac acted as continuous phase was a substrate for stearic acid. Tominimize the surface energy in the process of solvent evaporation,stearic acid had to aggregate, resulting in gaps for SSA5 and SSA20.Moreover, the evaporation of stearic acid at a very low pressurealso cannot be ruled out, because vacuum reached in sputteringprocess is 1 × 10−3 Pa. For SSA40 and SSA60 (Figs. 2i and j), it wasfound that the cross section of them exhibited concave structurewhich confirmed phase separating with heterogeneous structure,suggesting that shellac and 40 wt% or 60 wt% stearic acid could notmix sufficiently. The results were consistent with the conclusionsfrom the FTIR results.

Fig. 3 shows the FE SEM image of the cross section of thecoated film. The gray region of about 10 �m thickness on the topis the SSA20 coating layer, and the light region is dKGM substrate.The interface between coating and dKGM appeared to be an inti-mate adhesion. However, without observing the clear boundaryregion between the coating layer and dKGM, the coating did notpenetrate deeply into the dKGM substrate, indicating that the inter-action at the interface belonged to physical interaction. The resultconformed with the report in the literature for efficient bond-ing mechanisms of immiscible polymers (Jannerfeldt, Boogh, &

Månson, 2000). Furthermore, the FT-IR spectra of the dKGM andcoated films (Fig. A.1 in Appendix A) also indicated that only physi-cal interaction existed between coating layer and dKGM (Cao et al.,2006; Lu & Zhang, 2002; Lu et al., 2004).

122 X. Wei et al. / Carbohydrate Polymers 118 (2015) 119–125

Fig. 2. FE SEM images of surface (top) of the coated films for dKGM-SSA0 (a), dKGM-SSA5 (b), dKGM-SSA20 (c), dKGM-SSA40 (d), dKGM-SSA60 (e) and the cross section(bottom) of the coating for SSA0 (f), SSA5 (g), SSA20 (h), SSA40 (i), SSA60 (j). Insets show the photographs of the water contact angle on each surface after 30 s: dKGM-SSA0(68.0 ± 0.15◦), dKGM-SSA5 (72.1 ± 2.6◦), dKGM-SSA20 (92.0 ± 2.2◦) and dKGM-SSA40 (101.2 ± 1.6◦). Scale bars of (a)–(e): 30 �m; scale bars of (f)–(j): 10 �m.

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ig. 3. FE SEM images of the cross section of dKGM-SSA20. Scale bars: 30 �m.

The optical transmittance (T) curves of the dKGM and coatedlms are shown in Fig. 4. The transparency of the films is an aux-

liary criterion to judge the miscibility of the composite materialsCao et al., 2006). The interface between two materials will causeosses in optical transmittance, due to the quantity of light that isormally scattered and reflected at the interface of the differentolid materials (Lu et al., 2004). Interestingly, with an increase oftearic acid content up to 5 wt%, the T values of the coated filmsere much higher than that of dKGM, and increased nearly to

0%, but then decreased gradually. Enhancement of T values ofhe coated films reflected that the adhesion between the coatingayer and dKGM was intimate, and the mixture between shellacnd stearic acid in the coating layer was uniformly, which can bexplained by the effect of gaps uniformly dispersed throughouthe coating layer. Apparently, the T value of dKGM-SSA60 film was

uch lower than that of dKGM-SSA5, which may be attributed to

he phase-separated structure in the coating layer. This result wasn a good agreement with the conclusions from the FTIR resultsnd FE SEM micrograph. In addition, all the coated films showed

Fig. 4. The optical transmittance (T/%) curves of the dKGM and coated films.

obvious ultraviolet absorption capacity at 200–300 nm, indicatingthe potential application in food packaging.

3.2. Moisture barrier properties

3.2.1. Water vapor permeabilityTable 1 shows the WVP of the dKGM, the coated films and PE

films which were used as reference for comparison of moisture bar-rier efficiency. The RH and temperature gradients chosen for thismeasurement were 57–99% and 23 ◦C; 11–99% and 23 ◦C; 57–99%and 38 ◦C. Tested at 23 ◦C, the WVP values of the coated filmsnotably decreased with an increase of the stearic acid concentrationin the coating layer, reaching the values of 0.60 × 10−11 g/(m s Pa)(at 57–99% differential); 0.79 × 10−11 g/(m s Pa) (at 11–99% differ-ential), respectively, for dKGM-SSA20, which had no significantlydifference with dKGM-SSA40 and dKGM-SSA60. This could beexplained that the coating layer with higher stearic acid con-centration had much better moisture resistance, owing to thehydrophobicity of stearic acid. In addition, the WVP values

tion in the coating layer at 38 C, but were obviously higherthan that of 23 ◦C. This may be attributed to the thermallyunstable of shellac whose softening temperature was at 53.9 ◦C

X. Wei et al. / Carbohydrate Polymers 118 (2015) 119–125 123

Table 1Water vapor permeability at 23 ◦C (�RH:57–99%; �RH: 10–99%) and 38 ◦C (�RH:57–99%) of the dKGM and coated films, compared to PE film.

Samples Thickness (�m) Water vapor permeability [10−11 g/(m s Pa)]

�RH:57–99%, 23 ◦C �RH:11–99%, 23 ◦C �RH:57–99%, 38 ◦C

dKGM 38.40 ± 1.40 13.55 ± 1.20 22.48 ± 2.32 33.55 ± 1.88dKGM-SSA0 36.83 ± 2.84 2.63 ± 0.25 a 4.57 ± 0.38 a 10.95 ± 0.92 adKGM-SSA5 37.47 ± 0.83 1.20 ± 0.27 b 2.70 ± 0.19 b 4.47 ± 0.72 bdKGM-SSA20 37.73 ± 2.53 0.60 ± 0.07 c 0.79 ± 0.19 c 2.30 ± 0.45 cdKGM-SSA40 38.40 ± 2.42 0.37 ± 0.04 c n.d. 1.31 ± 0.32 cdKGM-SSA60 38.60 ± 0.53 0.32 ± 0.10 c 0.58 ± 0.23 c 2.22 ± 0.43 c

1 0.14 ± 0.03 0.24 ± 0.03

n each other according to Duncan’s Multiple Range Test (P < 0.05).

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Luangtana-Anan et al., 2007). Although the moisture barrier effi-iency of the coated films were still lower than that of PE films0.14 × 10−11 g/(m s Pa), 11–99% RH at 23 ◦C] which close to others

easurement [0.19 × 10−11 g/(m s Pa), 22–84% RH at 25 ◦C] (Phanhe et al., 2008), the high WVP of dKGM films were remark-bly reduced by coating shellac/stearic. With thickness of about8 �m, the coated films in present work were comparable to theilayer membranes obtained by depositing a layer of shellac (withhickness of 27.67 �m) onto the surfaces of dried preformed agar-37.38 �m) or cassava starch-based layers (47.77 �m) (Phan Thet al., 2008). Therefore, this work provided a convenient way torepare thin moisture-resistant films.

.2.2. Water uptakeTo further investigate the water-resistance of the coated films

ompared with dKGM, the films were soaked in water and theynamic water uptake plot was measured. The water uptake ofhe coated film and dKGM immersed in distilled water is shown inig. 5. For dKGM, the saturated water uptake ratio reached nearly00%. As was predicted, the coating layer could reduce the waterptake ratio because of the moisture-resistant buffering effect ofhe shellac and stearic acid. Moreover, this effect became moreignificant with an increase of the stearic acid concentration, andhe saturated water uptake ratio for dKGM-SSA20 was about 135%.herefore, the water-resistance ability was improved as a result ofhe presence of the coating layer.

.2.3. Surface hydrophobicityPhotographs of water drop deposited on the surface of the dKGM

nd coated films are shown in Fig. 2. It illustrated that the waterad a low affinity for the surface of the coated films, which had a

ig. 6. The time dependence of the water content of wheat flour stored in vessels sealedheat flour stored in vessels sealed with dKGM-SSA40 (a–b) and PE films (c–d) for 0 (a an

Fig. 5. Water uptake of the dKGM and coated films.

relatively high hydrophobicity. The contact angle of dKGM-SSA40reached a maximum value of 101.2◦, which higher than that ofdKGM (92.4◦). This result further verified that the coated films hadbetter water resistivity than dKGM. It is noted that the contactangles of the coated films increased with an increase of the stearicacid concentration in the coating layer, resulting from higher den-

sity of stearic acid in the coating layer. To further demonstrated thecontribution of shellac/stearic acid coating to the improvement ofthe hydrophobicity, a filter paper was coated with shellac/stearicacid also photographed (Fig. A.2 in Appendix A). Therefore, the

with the coated films and PE films at 34 ◦C and 99% RH (left) and photographs ofd c) and 28 d (b and d) at 34 ◦C and 99% RH, respectively (right).

124 X. Wei et al. / Carbohydrate Poly

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See Figs. A.1 and A.2.

ig. 7. Stress–strain curves (a) and effect of SA content on tensile strength (�b),longation at break (εb) and Young’s modulus (E) of the coated films (b).

oating layer consisted of shellac and stearic acid significantlymproved the hydrophobicity of the coated films.

.2.4. Practical application: storage of wheat flour in the coatedlms

Fig. 6 shows the time dependence of the water content (Fig. 6,eft) and the photographs (Fig. 6, right) of wheat flour stored inessels sealed with the coated films and PE films during the storedime. From the curves, water content of wheat flour increased overime and reached almost the same maximum value of 16% at 28

both for the coated films and PE films, which implied that theoisture barrier efficiency of the coated film was comparable to

hat of PE. There were no moldy wheat flour observed during 28 dFig. 6, right). Therefore, the coated films had potential to be usedn moisture-resistant package materials.

.3. Mechanical properties

Fig. 7 shows the mechanical properties of the coated films andKGM. The tensile strength (�b) and elongation at break (εb) ofKGM was 36.2 MPa and 9.0%, respectively (Fig. 7a). While, the

oated films dKGM-SSA0 which only contained shellac in the coat-ng layer was 31.0 MPa, 1295 MPa and 13.3%, respectively (Fig. 7and b), indicating that the shellac in the coating layer contributedo the enhancement of εb, but failed to enhance the �b and

mers 118 (2015) 119–125

E of the coated films. However, the �b and E values increasedsimultaneously with the increase of the stearic acid content andreached maximum values of 41.0 MPa and 2069 MPa, respectively,for dKGM-SSA20 (Fig. 7a and b). It was mainly attributed to theuniform mixing between shellac and stearic acid in the coatinglayer. This also further confirmed that the shellac had intimateadhesion on the surface of dKGM, allowing coating layer to playa role in enhancement of mechanical properties. However, otherstudy reported that the lipid layer such as acetem could increasethe elongation for whey protein isolate film but had a negligiblemechanical strength (Anker et al., 2002), which further confirmedthe interaction effect of shellac and stearic acid on the enhancementof mechanical properties in our work.

4. Conclusions

The moisture-resistant konjac glucomannan films were success-fully prepared by coating shellac/stearic acid with about 10 �mthickness to deacetylated konjac glucomannan films. When stearicacid content in the coating layer increased 5 up to 40 wt%, the mois-ture barrier properties and mechanical properties of the coatedfilms were much improved compared with those of the dKGM anddKGM-SSA0 films. It was noteworthy that the coated film with20 wt% stearic acid in the coating possessed the highest tensilestrength of 41.0 MPa and modulus (2069 MPa). Shellac in the coat-ing layer contributed to improving the optical transmittance of thecoated films. Stearic acid in the coating played an important rolein enhancing moisture barrier properties and mechanical proper-ties of the coated films. The key factors for the enhancement ofall the properties of the coated films should be attributed to theintimate interfacial adhesion between the coating layer and dKGMsubstrate, and the uniform mixing between shellac and stearic acidin the coating layer.

Acknowledgements

This work was supported by the Natural Science Foundation ofShandong Province (2012ZRC21003) and National Natural ScienceFoundation of China (31271837).

Fig. A.1. FTIR spectra of the dKGM film, shellac (SSA0), and the coated films.

X. Wei et al. / Carbohydrate Poly

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